WhatsApp and Facebook are owned by the same company.
Facebook MIGHT know your phone number, it might not. It also might know the phone numbers of your friends. It depends on what you and they have allowed it to know. If you have Facebook on your phone it's quite likely that you ticked "yes" to something without reading it and Facebook already knows your number.
WhatsApp DOES know your phone number, and the phone number of the people you contact on WhatsApp, because that's how it works.
Soon you are going to get an update to WhatsApp on your phone. You will probably click the "yeah whatever" button.
If you do that, WhatsApp and Facebook will join up their information about your phone number and the phone number of your WhatsApp contacts and the phone numbers of your Facebook friends. Their spider diagram of "who knows who" will get much better and more interconnected.
If you don't want them to do that, when you get the update message you can tap on "more information" and say "no".
If you already got the update and tapped on the "yeah whatever" button, you can go to WhatsApp settings, Account, and say "no" to sharing your account info.
You've got 30 days to say no, and after that WhatsApp and Facebook will do their big database mergey thing (which can't be then undone).
The Verge article